Geophysical field gradient surveys generally are conducted by using line scans which give results along that line. Where an area is to be surveyed a number of line scans, preferably parallel, are made and interpolations of data between those line scans are made. The data gathering process using airborne sensors is very expensive. While this has been carried out during the last twenty years for marine gravity gradients, it is only in the last three years that commercial services in this field have been offered for airborne. Similarly, commercial services for airborne magnetic gradients are now being offered.
However, present methods do not make the best use of the information and there is a need for improved estimation and visualisation of geological features. To obtain a visual representation of the feature, for example on a computer screen, the intensity of the observed field is usually rendered as a map of coloured pixels at specified X-Y positions on the screen. The colour of the pixel relates to the intensity of the subject field. The X-Y positions on the computer screen relate to specific geographical grid points on the earth's surface. The subject tensors are derived from instrument readings taken at geographical positions which usually do not coincide with the required grid points. Since it is necessary to calculate the intensity of the grid points relating to the display, it is necessary to interpolate between the observed values to obtain the values at the required grid points. If the interpolation can be improved, previously unseen geological features may now be seen on the resultant coloured map. At present interpolation is effected using weighted linear methods or other well known means, using only the raw data. The rotational component of the raw data is not isolated. Ultimate users of the data would be unaware that much valuable information was lost thereby. As gatherers of data are unaware that much of the noise of the signal reports in the rotational component, they may neglect to gather the full gradient data in the first place.
While the separation of structural and rotational tensors can be obtained by hand using graphical methods such as “Mohr's Circles” which are well known in the rock mechanics field, those methods are far too slow to be useful for the volumes of data involved in geophysical surveys, especially airborne geophysical surveys.
It is an object of the present invention to ameliorate disadvantages referred to above.